Re: How tall were the Appalachian Mountains at the time of their formation?

This is a difficult question to answer and is still the subject of some 
controversy.

The tallest peaks in the current Appalachians have a relief of about 2000 
meters.  This sets a minimum boundary, since presumably today's relief is 
less that that in the past, as a result of erosion.  However, there is also 
the suggestion of recent uplift in some areas of the Apaalachians, which 
might be creating youthful relief.

For more on Appalachian geomorphology (landform shape and development), 
see:
 http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_2/GEO_CHAPTER_2_TABLE
.HTML and look at plates 11, 12, and 13 along with their captions.

Also, a search for "paleorelief" on GeoRef should be useful.  Using that 
keyword on the Geological Society of America publications database 
(http://www.geosociety.org/pubs/db-query.htm)turned up this paper:

Eusden, J.D., Jr., and Lux, D.R., 1994, Slow late Paleozoic exhumation in 
the Presidential Range of New Hampshire as determined by the 40Ar/39Ar 
relief method: Geology, v. 22, no. 10, p. 909-912.

They find that the current 1900 m height of Mt Washington is all post-247 
Ma relief (post-Alleghenian, possibly rift-related or related to the White 
Mountains hot spot trace).

A reasonable upper bound on paleotopography can be set by looking at 
collisional orogenies 
today, namely the Alpine-Himalayan orogeny.  The highest peaks of these 
mountains range from 5000 to 8000 meters high and the 8000 meter high peaks 
of the Nepal and Bhutan Himalayas are collapsing gravitationally at the 
same time they are being thrust upward (Mt Evrest, for example is cut by a 
huge normal fault that is riding in the upper plate of the Main Central 
Thrust and both are active at the same time), so 8000 meters is probably as 
high as a mountain on earth can ever get.

To do more detailed calculations, you could look at such things as the size 
of the Appalachian foreland basins as a way to estimate flexural loading 
and erosion rates, fossil pollen species as a measure of elevation zones, 
reconstruction of Appalachian structures.  I didn't find anything on my 
quick look-around, but, as a grad student, you should have access to GeoRef 
or GeoBase and should be able to pursue this question into the primary 
literature.  The main problem is that elevation, relief, and exhumation 
are all a little different and not easily comparable.  Good luck.

Dave Smith
Geology and Environmental Science
La Salle University, Philadelphia, PA 19141
dsmith@lasalle.edu